1. Field of the Invention
This invention relates generally to a fuel cell for a fuel cell stack and, more particularly, to a fuel cell for a fuel cell stack, where the fuel cell includes nested flow channels in an active region of the fuel cell and non-nested flow channels in inactive feed regions of the fuel cell, and where the diffusion media layers in the cells are removed in the inactive feed regions to provide more space for the non-nested channels.
2. Discussion of the Related Art
Hydrogen is an attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid-polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of flow field plates or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided in the anode side of the bipolar plates that allow the anode gas to flow to the anode side of each MEA. Cathode gas flow channels are provided in the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of each MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells from one cell to the next cell as well as out of the stack.
It has previously been proposed by the inventors in U.S. patent application Ser. No. 10/661,195, titled Nested Stamped Plates for a Compact Fuel Cell, filed Sep. 12, 2003, that the thickness or repeat distance of a fuel cell stack can be reduced by nesting the flow channels in the active region of the fuel cells. FIG. 1 is a cross-sectional view of a portion of a fuel cell stack 10 showing this proposed design. The fuel cell stack 10 includes two MEAs 12 and 14 for adjacent fuel cells in the stack 10. Each MEA 12 and 14 includes a membrane of the type referred to above and an anode side catalyst layer and a cathode side catalyst later. An anode side gas diffusion media layer 16 is positioned adjacent to the MEA 12 and a cathode side gas diffusion media layer 18 is positioned adjacent to the MEA 14. The diffusion media layers 16 and 18 are porous layers that provide for input gas transport to and water transport from the MEAs 12 and 14. Various techniques are known in the art for depositing the catalyst layers on the membranes in the MEAs 12 and 14 or on the diffusion media layers 16 and 18.
A bipolar plate assembly 20 is positioned between the diffusion media layers 16 and 18. The bipolar plate assembly 20 includes two stamped metal bipolar plates 22 and 24 that are assembled together in the nested configuration as shown. The nested plates 22 and 24 define parallel anode gas flow channels 28 and parallel cathode gas flow channels 30, where the anode flow channels 28 provide a hydrogen flow to the anode side of the MEA 12 and the cathode flow channels 30 provide airflow to the cathode side of the MEA 14. Additionally, the plates 22 and 24 define coolant flow channels 32 through which a cooling fluid flows to cool the fuel cell stack 10, as is well understood in the art. In this design, the size of the coolant flow channels 32 is reduced from the size of the cooling channels provided in the non-nested stamped plates of the prior art, which provides the reduction in the repeat distance of the fuel cell stack 10. Reducing the size of the coolant flow channels 32 over the known cooling channels does not significantly affect the cooling capability of the cooling channels because the larger channels were more than adequate to provide the necessary cooling. The reduction in coolant volume also reduces the thermal mass that must be heated during system start-up.
The anode flow channels 28 are in fluid communication with an anode flow channel header at each end of the fuel cell stack 10, where one header receives the anode gas flow to distribute it to the anode gas flow channels 28 and the other anode header receives the anode exhaust gas from the anode flow channels. Likewise, the cathode gas flow channels 30 are in fluid communication with a cathode flow channel header at each end of the stack 10, and the cooling flow channels 32 are in fluid communication with a coolant flow channel header at each end of the stack 10. However, in order to couple the anode flow channels 28 to the anode channel headers, the cathode flow channels 30 to the cathode channel headers and the coolant flow channels 32 to the coolant channel headers, it is necessary to separate and un-nest the plates 22 and 24 in the non-active feed regions of the stack.
Because the non-nested configuration of the flow channels 28, 30 and 32 requires more space than the nested configuration of the channels 28, 30 and 32, the reduction in thickness of the stack 10 provided by the nested configuration would be eliminated by using the known non-nested configuration in the inactive regions. It is possible to reduce the size of the flow channels 28, 30 and 32 in the non-nested inactive regions so that the flow channels 28, 30 and 32 do not use more space than they use in the nested configuration. However, such a reduction in the size of the channels 28, 30 and 32 would cause a pressure drop across the channels that would adversely affect the flow rate and performance of the stack 10.
The present invention proposes a solution to a transition from the nested configuration to the non-nested configuration of the bipolar plates without reducing the size of the channels or increasing the thickness of the stack.